Why some still images trigger seizures

his figure depicts how that same visual patterns that elicit a specific type of brainwave (gamma oscillation) in healthy visual cortex can also trigger seizures in patients with photosensitive epilepsy. Image credit: Hermes et al.

It’s not just flashing lights that pose a danger to people with photosensitive epilepsy. Still images can also trigger seizures.

People with photosensitive epilepsy already know to avoid flashing lights. But seizures can also be triggered by still images. The visual patterns to blame can also cause migraines in those with general photosensitivity, and make even healthy people feel a little woozy. In order to work out why certain images have this effect, researchers conducted an extensive review of existing data on how patterns affect the brain. The findings could inform better building designs, making the built environment safer for people with photosensitive epilepsy, and more comfortable for everyone else.

Hermes: Looking at certain visual patterns can trigger seizures in patients with photosensitive epilepsy, even if the images are static. We recently found that looking at different visual patterns of high-contrast black and white stripes or noise patterns (basically clouds) produce very different effects on gamma oscillations compared to other brain signals, such as neuronal firing and the fMRI (functional magnetic resonance imaging) signal. It was very surprising to find out that these stimuli can cause seizures in humans.

Why looking at these particular patterns can cause seizures is unknown. Our commentary, based on a review of epilepsy and neuroscience literature, proposes that the same patterns that trigger seizures in patients also cause oscillating brain waves in healthy visual cortex. These oscillations are at the time scale of about 30-80 cycles per second, and are called gamma oscillations. Our commentary aggregates data from several decades of literature and makes this proposal, but does not include new empirical data to test the idea further.

This figure depicts how that same visual patterns that elicit a specific type of brainwave (gamma oscillation) in healthy visual cortex can also trigger seizures in patients with photosensitive epilepsy. Left top: large, high-contrast grating patterns induce gamma oscillations in visual cortex. Right top: the same patterns that elicit gamma oscillations are also the most likely to evoke epileptiform activity and seizures in patients with photosensitive epilepsy. Left bottom: noise patterns do not elicit large gamma oscillations and are also unlikely to trigger epileptiform activity. Credit: Hermes et al.

RG: What motivated this study?

Hermes: An epilepsy neurologist mentioned that looking at a grating pattern can elicit a seizure in some patients. This rang a bell because of our previous work, and I reviewed the literature with Jonathan Winawer, an expert in the field of vision science, and Dorothée Kasteleijn-Nolst Trenité, a photosensitive epilepsy specialist. We discovered a large overlap between the types of patterns that produce gamma oscillations and the patterns that are most likely to provoke a seizure in pattern sensitive epilepsy.

RG: Where do people usually encounter these patterns?

Hermes: In the natural setting, blinds in front of a window are a good example of a provocative pattern. We do not yet know enough to use this description to predict whether a particular natural image, like a city scene or a portrait, is more likely or less likely to cause gamma oscillations, or seizures.

RG: Who is affected by these types of images?

Hermes: Only patients with photosensitive epilepsy have seizures from looking at these types of images. However, general photosensitivity without seizures can be found in 0.3-3 percent of the population. Many of these people may have visual discomfort, or even migraines, triggered by the types of images that also induce strong gamma oscillations. Even perfectly healthy people may feel modest discomfort from the images that are most likely to trigger seizures in photosensitive epilepsy.

RG: What applications do your findings have in preventing seizures?

Hermes: Our findings imply that gamma oscillations may be a potential biomarker of photosensitivity. It is very important to recognize photosensitive epilepsy early: a seizure is potentially life-threatening and may require a visit to the emergency department. Early recognition and removing potentially provocative stimuli from a child’s environment can limit the number of seizures.